Lesson 14: Black body radiation

Question 1

True or False

Only hot objects emitt infrared radiation and only cold objects absorb infrared radiation.

Answer 1

• False.
• All bodies (objects), no matter what temperature, emitt and absorb infrared radiation.

Question 2

If an object/ body is hot, how does this affect the amount of infrared radiation emitted from the object/ body in a given time?

Answer 2

• The hotter the object, the more infrared radiation it radiates in a given time.

Question 3

What is a “perfect black body?”

Answer 3

• A perfect black body is an object that absorbs all of the radiation incident on it.

Question 4

True or False

A black body will absorb, reflect and transmitt any incident radiation on it.

Answer 4

• False.
• A black body does not reflect or transmitt any radiation.

Question 5

True or False

A perfect black body is only a good absorber of infrared radiation.

Answer 5

• False.
• A perfect black body is a good absorber AND emitter of infrared radiation.

Question 6

What is the emitting/ absorbing of an object at a constant temperature like?

Answer 6

• The object is absorbing infrared radiation at the same rate as it is emitting radiation.

Question 7

When does the temperature of a body increase - in terms of the relative amount of infrared radiaition absorbed/ emitted by the body?

Answer 7

• The temperature of a body increases when the body absorbs radiation faster than it emitts radiation.

Question 8

What 2 factors does the temperature of the earth depend on?

Answer 8

1.) The rates of absorption/ emission of radiation.
2.) Reflection of radiation into space.

Question 9

How does the intensity/ wavelength of infrared radiation emitted from an object/ body depend on the temperature of the object/ body?

Answer 9

• Very hot objects emitt shorter -wavelength radiation than cooler objects.
• The intensity of radiation emitted from hot objects is greater than the intensity of radiation emitted from cool objects.

Question 10

When earth absorbs short wave-length radiation from the sun, what happens to this radiation once it has been absorbed by the earth?

Answer 10

• The radiation is reflected back into space.
• Some of the energy (reflected off) is trapped by greenhouse gases in the atmosphere.

Question 11

What is happening due to an increase of greenhouse gases in the earth’s atmosphere?

Answer 11

• More energy is trapped in atmosphere.
• Less energy is radiated into space.

Question 12

If an object (ie a hot cup of tea) is hotter than its surroundings - what is the radiation emitted/ absorbed by the object like ?

Answer 12

Hotter object emits more radiation than it absorbs - decreasing its temperature.

Question 13

If an object is cooler than its surroundings, what is the absorption and emission of infrared radiation of the object like?

Answer 13

The object absorbs more radiation than it emits.